1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to a data driver IC of a liquid crystal display and a driving method of the data driver IC.
2. Description of the Related Art
Recently, liquid crystal displays (LCDs) are becoming widely used as an apparatus for displaying various images including a still picture or a moving picture, and their applications are expanding due to improvements in picture quality achieved by using an improved liquid crystal material and the development of better pixel processing techniques. LCD displays also have the advantages of light weight, thin profile, and low power consumption.
An active matrix LCD (AM-LCD) generally includes an array substrate including a lower substrate of a liquid crystal panel for image display. On the array substrate, a plurality of pixels are arranged in a matrix configuration, a plurality of thin film transistors (TFTs) functioning as a switching element are also formed, and a plurality of gate lines and a plurality of data lines crossing the plurality of gate lines are arranged.
FIG. 1 is a block diagram of a related art AM-LCD.
Referring to FIG. 1, the related art AM-LCD includes a data driver IC 3 for providing a liquid crystal panel 6 with image data from an external video card 1, a gamma voltage IC 4 for supplying a signal voltage to the data driver IC 3, a gate driver IC 5, which provides the liquid crystal panel 6 with a scanning signal for controlling the switching of thin film transistors of the liquid crystal panel 6, and a controller 2 for controlling the data driver IC 3 and the gate driver IC 5.
The liquid panel 6 with an XGA resolution (1024*768 pixels) level has 1024*3 (one for each of red, green, and blue) data lines. For instance, in an LCD with XGA resolution, 8 data driver ICs each having output terminals of 384 channels, and 4 gate driver ICs each having output terminals of 200 channels are used.
Video data provided by a video card built in a computer or another source is supplied to the data driver IC 3 via the controller 2. In another example, an analog image signal input by a computer is converted to digital video data through an interface module built in an LCD monitor, and the converted digital video data is input into the LCD.
The gate driver IC 5 applies a scanning pulse once per frame period to each scanning line, and timing of the scanning pulse is interlaced from an upper side of the liquid crystal panel toward a lower side. The data driver IC 3 applies a liquid crystal driving voltage, which corresponds to pixels of one row to which the scanning pulse is applied, i.e., a signal voltage to each data line.
In selected pixels to which the scanning pulse is applied, corresponding thin film transistors are turned on as voltages of corresponding gate electrodes connected to the gate line increase.
The liquid crystal driving voltage is applied to the liquid crystal from the data line via the drain electrode and the source electrode of each of the thin film transistors, so that each of the corresponding pixels charges a pixel capacitance corresponding to a sum of the liquid crystal capacitance and the storage capacitance. By repeating the operation, a voltage corresponding to an image signal is applied to a pixel capacitance of the liquid crystal panel repeatedly per frame
FIG. 2 is a block diagram of the data driver IC of FIG. 1. In FIG. 2, it is assumed that external digital data has 6 bits for the convenience of this description.
Referring to FIG. 2, all bits of external digital data D5, . . . , D0 are sequentially sampled by a shift pulse in a first latch 22, and when the sampling ends, the data are transferred to a second latch 24 by a line pass signal at a time.
Next, the sampled digital data are converted into an analog image signal by a digital-to-analog converter (DAC) 26. The converted analog image signal is amplified by an amp, and transferred to data lines 28 of the liquid crystal panel.
In the related art LCD, a driving IC board, i.e., a board provided with a data driver IC and a gate driver IC, is installed separately from the liquid crystal panel.
However, as low temperature polysilicon is developed, allowing use of large-sized glass substrates, and the integration technology of driver ICs advances, it becomes possible to integrate circuits for processing the display signals on the glass substrate. In addition, the variety of circuits that can be integrated on the glass substrate increases.
In other words, the above system-on-LCD includes polysilicon TFTs formed on a glass substrate for use in both the pixel array and the driver IC of the liquid crystal panel. By doing so, the cost of fabricating a module in the fabrication of an LCD can be saved and the power consumption of the LCD is reduced.
FIG. 3 is a circuit diagram of a unit pixel region in a related art liquid crystal panel.
Referring to FIG. 3, a gate line 31 and a data line 32 crossing the gate line 31 are formed on a glass substrate. A thin film transistor (TFT) 33 connected to the gate line 31, and the data line 32 is formed near a crossing point of the gate line 31 and the data line 32. A drain electrode 33d of the TFT 33 is electrically connected with a pixel electrode of a liquid crystal cell 38.
An auxiliary capacitance 35 for maintaining a voltage of the pixel electrode 37 during one field period is also formed between the TFT 33 and the pixel electrode 37. One terminal 34 of the auxiliary capacitance 35 is connected to the drain electrode 33d of the TFT 33, and the other terminal 36 is connected to a common voltage Vcom commonly applied to the pixel electrode 37.
When a gate signal is applied to the gate line 31, the TFT 33 is turned on as aforementioned, and the analog image signal is transferred to the pixel electrode 37 via the data line 32 and maintained by the auxiliary capacitance 35. The image signal voltage transferred to the pixel electrode 37 is applied to the liquid crystal cell 38, thereby aligning the liquid crystal.
In the related art LCD, an image can be obtained regardless of whether the image is from a moving picture or a still picture. As an example of the still picture, a battery indicator image for displaying the power remaining in a battery of a handheld terminal is displayed on an LCD of the handheld terminal.
However, in the related art LCD, displaying the still picture, like displaying the moving picture, requires turning the TFT on using a gate signal and again inputting an image signal to each pixel electrode. To this end, the gate driver IC, the data driver IC, and a controller for controlling an operation timing of the driver ICs is always working, which increases the power consumption.
Accordingly, an LCD having a static memory device provided at each pixel has been used.
FIG. 4 is a circuit diagram of a unit pixel region of a liquid crystal panel having a related art maintenance circuit.
Referring to FIG. 4, pixel electrodes 37 are arranged in a matrix configuration on a substrate. For the convenience of description, only one pixel electrode is shown in the drawing. Data lines 32 are arranged in left and right sides of the pixel electrode, and gate lines 31 are arranged in upper and lower sides of the pixel electrode 37.
The maintenance circuit 42 utilizes a memory in which two-stage inverters are in a positive feedback arrangement. In other words, a static random access memory (SRAM) may be used as a maintenance circuit for the digital video signals. Particularly, unlike DRAM, SRAM is appropriate because it does not require a refresh operation for maintaining data.
To reproduce a still picture in 8 colors with the LCD of FIG. 4, only the SRAM added to the unit pixel operates. Accordingly, the driver ICs and an external module are inactivate, providing the liquid crystal panel with a low power consumption characteristic.
However, in the LCD having the unit pixel of FIG. 4, because the number of devices constituting the memory is high, apparatuses and processes capable of using a simplified design are needed. Also, although such an LCD can be achieved, it cannot be used in the transmission type LCD because it needs a layout throughout the whole area of the pixel regions.